The past few years has witnessed the ever-increasing availability of relatively cheap, low power wireless data communication services, networks and devices, promising near wire speed transmission and reliability. One technology in particular, described in the IEEE Standard 802.11a (1999) and Draft IEEE Standard 802.11g (2002) High Rate PHY Supplements to the ANSI/IEEE Standard 802.11, 1999 edition, collectively incorporated herein fully by reference, has recently been commercialized with the promise of 54 Mbps effective bandwidth in the less crowded 5 GHz band, making it a strong competitor to traditional wired Ethernet and the more ubiquitous “802.11b” or “WiFi” 11 Mbps wireless transmission standard.
IEEE 802.11a and 802.11g compliant transmission systems achieve their high data transmission rates using OFDM encoded symbols mapped up to 64 QAM multicarrier constellation. Before final power amplification and transmission, the multicarrier OFDM symbol encoded symbols are converted into the time domain using Inverse Fast Fourier Transform techniques resulting in a relatively high-speed time domain signal with a large peak-to-average ratio (PAR).
The large PAR characteristic of this transmission signal makes it difficult to use sub-class A RF power amplification without significant back-off due to nonlinear effects of such power amplifiers, which reduces generated signal strength, effective range, and, ultimately utility as a wireless transmission system. Class A amplifiers are too power inefficient market for mobile users where wireless transmission has the highest penetration, so ways to extend the linear response of more power efficient sub Class A power amplifiers are currently being explored.
Known techniques to extend the nonlinear performance of sub Class A amplifiers when faced with amplifying high-speed, high PAR signals include digitally clipping and otherwise compressing the PAR values of such signals. This brings up the effective transmission gain up 2-3 db typical since PAR is compressed, but still does not provide sufficient extended range over non-implementing systems and can indirectly reduce effective throughput in IEEE 802.11a & 802.11g compliant systems, because such systems will reduce transmission rates in an effort to compensate for reception errors in fringe reception environments.
Therefore, the wireless industry has turned to adaptive predistortion in an attempt to actually extend the linear gain and phase response of power efficient Class AB and other designs. Known adaptive predistortion techniques compare the output of the power amplifier against the input signal to determine e.g. gain and phase nonlinearities between the two, create an predistortion correction function to process the input signal to counteract those nonlinearities when they are experienced. Typically, a predistorter using a signal processor, lookup table, or a combination thereof interposes the input and the amplifier to implement the adaptive predistortion.
One obstacle to implementing a successful predistortion design with respect to high-speed, high-PAR signal transmission as required by the IEEE 802.11a &g physical layer standards has been the issue of accounting for the delay it takes to self-receive the output of the power amplifier after a given input signal has been fed to the predistorter. Note here that in order for adaptive predistortion to be successful, it is important that the output signal at the power amplifier be compared to its corresponding input signal to great temporal precision. Accounting for this delay still appears to be a black art fraught with trial-and-error, since it appears that designers simply approximate the delay from the input to the predistorter to the output of the self-receiver based on implementing component delays and then verifying and tweaking their designs through trial-and-error until the experienced delay is found and accommodated. Further, this design approach appears to disregard or minimize the importance of changing power levels and frequencies of the input signal which may alter the self-receive delays, as well as other potential delay altering issues, such as component aging, environmental effects, and interference.